The invention relates to a method of manufacturing molded parts, such as automobile interior trim components, that have a molded-in surface texture, as well as an apparatus for carrying out such a method.
In various technical fields, and particularly in the automobile manufacturing industry, there is a constantly growing demand for various molded parts that have a surface texture molded into or onto the surfaces of such parts. For example, it is increasingly desired to provide interior trim components of a motor vehicle, such as headliners, door trim insert panels, column trim covers, consoles, parcel shelves, dashboards, and the like, with a surface texture such as an artificial leather grain, an artificial wood-like grain, a pattern of dots or lines or the like, simulated stitching lines, raised bumps and indented depressions or even molded-in text or logos. Such surface textures enhance the appearance of the visible surface of the molded part, or enhance the surface feel or grip characteristic provided by the surface, or provide intended information in the way of text or logos or the like.
In any event, however, the molded-in texture must be accurately and consistently represented or reproduced on the surface of the molded part, in order to achieve the intended effects. On the other hand, if the intended surface texture is inaccurately or distortedly reproduced on the surface of the molded part, this results in a visually unacceptable part which must be scrapped. Thus, it has been a significant problem in the industry, that defective or unacceptable surface texturing leads to a rather high reject and discard rate of such parts, although the parts may otherwise be fully in conformance with other requirements.
If has also been extremely difficult or impossible up to the present time, to achieve complex, or finely detailed surface textures, without distortion or misrepresentation, using conventionally known methods. One conventional method involves providing a flat starting sheet of cover stock that is pre-textured with the desired surface texture. Then, this pre-textured cover stock or cover sheet is deep-drawn and molded while being laminated with a backing substrate or the like. During this molding operation, which forms three-dimensional contours of the finished molded part, the pattern or surface texture of the cover sheet is necessarily distorted at different areas, depending on the degree of stretching and three-dimensional molding that has taken place at these respective areas. Namely, an area that is more deeply or extensively drawn and stretched will suffer an area expansion of the intended surface texture. Thus, a regular geometric pattern of dots or lines or the like on the starting cover sheet material will clearly show the areas of distortion resulting from the three-dimensional molding, and will generally not be acceptable. Text or the like can also not be provided at molded areas. Such a technique is generally only suitable for random textures of which a distortion is not readily apparent, or for providing a texture on areas that are not strongly contoured. Another problem of such a conventional process arises when the textured surface of the cover sheet is pressed in contact with a flat surface of a mold. In such a case, the surface texture will be at least partially crushed or flattened or xe2x80x9cironed outxe2x80x9d, so that any resulting surface texture cannot have a very strong or roughly profiled texture.
To avoid the above-mentioned distortion that arises when deep-drawing or molding a pre-textured cover sheet, techniques have been developed to apply a surface texture onto a cover sheet or a surface of a molded part after the molding rather than before the molding operation. Such techniques are complicated, time consuming and not economical in production.
Two further conventional processes for forming a contoured skin sheet with a surface texture are the powder and spray slush methods. In the powder slush method, a thin galvanically fabricated zinc sheet is used as a contoured and textured mold which is heated and dipped into or coated with a polymer powder such as a polyvinyl chloride (PVC) powder, which then melts and forms a corresponding contoured and textured PVC skin on the zinc mold. The skin is peeled off, and can thereafter be applied onto a molded substrate. Alternatively, in the spray slush method, a liquid polymer such as polyurethane is sprayed onto the textured mold to form a textured skin, which can thereafter be peeled off and applied on a molded substrate. These slush skin methods are rather complicated, time consuming, and costly due to several additional required steps. Moreover, these processes generally use polyvinyl chloride and polyurethane materials, which are becoming ever more undesirable because they are not easily recycled, and they represent toxic risks and disposal problems. Also, the thin mold sheets have a relatively short useful life cycle, which leads to high tooling costs, and the surface textures that can be achieved are limited. Also, the conventional patterning or texturing processes all involve additional steps end additional work effort, beyond the molding operation itself.
In view of the above, it is an object of the invention to provide a method and an apparatus for producing a molded part having a surface texture that accurately represents the intended surface texture without distortion, and that is not limited as to the type of surface texture, but instead can be used to form synthetic leather grains, synthetic wood grains, patterns of dots and stripes or the like, raised bumps, indented depressions, synthetic stitching lines, text, logos, and the like with great detail, accuracy, and reproducibility. It is a further object of the invention to be able to mold the intended surface texture into the surface of the molded part during its molding fabrication, without requiring additional work steps or effort to achieve this surface texture. Another object of the invention is to use materials that are easily recyclable for the textured cover sheet, in combination with essentially any possible backing or substrate material. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification.
The above objects have been achieved according to the invention in a three-dimensionally contoured molded part including a substrate and a cover sheet laminated thereon, wherein the cover sheet includes a skin film and a foam backing that is adhered or bonded onto the substrate. The skin film is provided with a surface texture that is uniformly and consistently applied without distortion over three-dimensionally contoured areas of the molded part. Such a molded part may be produced, and the above objects have further been achieved according to the invention, in a method of molding a three-dimensionally contoured molded part having a surface texture.
The inventive method involves the following steps. A cover sheet is provided, which includes a surface skin film and a foam backing. The cover sheet is heated in such a manner so that the skin film is heated to at least its melting temperature while the foam backing is heated only to a temperature below its melting temperature. Thereby, particularly, the skin film material is entirely melted to a viscous liquid state, while the foam backing remains an elastic foam solid, whereby the foam backing acts as a solid carrier for holding and carrying the viscous liquid film. To some extent, the viscous liquid skin film material also penetrates into or partially saturates the open pores at the interface surface of the foam backing, somewhat like a liquid being absorbed into a sponge. However, since the foam backing is a closed-cell foam that is not permeable through its thickness, the extent of absorption of the viscous liquid skin film material into the foam backing is limited to the interface surface. Preferably the cover sheet is a foam-backed thermoplastic polyolefin (TPO) sheet material. Suitable thermoplastics for the skin film include polypropylene, polyethylene, polyvinyl chloride (PVC), and acrylonitrile butadiene styrene (ABS) polymers, for example.
The pre-heated cover sheet is arranged between a first or front mold and a second or back mold. The mold surface of the front mold has been provided with an exact negative image of the desired resulting surface texture of the finished molded part. Namely, this surface texture has been provided on the mold surface by mechanical engraving, embossing, chemical etching, or the like, after the surface of the mold has been milled and highly polished. After the pre-heated cover sheet has been arranged between the molds, the front mold and the back mold are closed relative to each other, while the back mold preferably mechanically pre-molds the cover sheet toward the front mold. An air-tight seal is provided between the front and back molds. A pressurized pressure medium is introduced into a gap between the back mold and the foam backing of the cover sheet, whereby the pressure medium further blow-molds the cover sheet toward and against the front mold. Air is vented out from between the skin film of the cover sheet and the textured surface of the front mold, and a low vacuum is applied between the skin film and the textured surface of the front mold.
In this manner, the pressurized pressure medium presses the cover sheet so that the skin film (in the melted viscous liquid state) is pressed uniformly and evenly into contact with the textured mold surface of the front mold, so as to mold the inverse texture into the skin film. Meanwhile, the low vacuum ensures that all air pockets between the skin film and the textured mold surface are removed, and that the skin film is held in fixed registration on the textured mold surface to avoid distortion or double imprinting of the texture image. Since the foam backing of the cover sheet is closed-cell foam that is non-permeable and that remains in an elastic solid (non-melted) state, there is no danger of a blow-through of the pressure medium, but instead, the foam backing acts as a molding buffer or intermediary layer to uniformly apply the molding pressure to the skin film to press it uniformly against the contoured and textured surface of the front mold.
Since the skin film is in a melted viscous liquid state as mentioned above, the skin film material is molded against the textured surface of the front mold, so as to accurately reproduce the opposite or inverse of the texture of the mold surface into the skin film surface. By then cooling against the front mold, the skin film re-solidifies with this surface texture fixed therein. Since the cover sheet is essentially simultaneously contoured and imprinted with the surface texture, there is no risk of the surface texture being distorted due to the contouring or molding of the cover sheet. Instead, any surface texture that can be engraved or etched or otherwise formed on the contoured mold surface can be accurately reproduced on the skin sheet.
In a subsequent step, the mold is opened, and the finished textured skin may be removed for further separate use, or a substrate can be formed or laminated onto the back surface of the foam backing of the cover sheet directly in the same molding apparatus. In a preferred embodiment, a pre-heated sheet of a substrate material (such as a composite of natural fibers, glass fibers or polyester fibers with thermoplastic fibers such as polypropylene fibers) is molded by the back mold against the back surface of the foam backing. According to further alternatives, a foaming polymer resin material may be injected, sprayed, poured or cast against the back surface of the foam backing, and then molded against the foam backing by the back mold. For example, in this regard, the substrate material could be a foaming polyurethane material. Preferably, the front mold and the back mold are so dimensioned and configured relative to each other, to accommodate the cover sheet therebetween, as well as a gap in the range of 2 to 5 mm, which acts as a pressure medium receiving gap during the initial molding of the cover sheet as mentioned above, and which will receive and form the substrate therein during the subsequent substrate laminating step.
Preferably, the pressure medium is compressed air which is provided at a gage pressure in the range from 1 to 30 bar and especially in the range from 5 or 6 to 20 bar. On the other hand, the low vacuum applied from the side of the front mold is substantially less than 1 bar below atmospheric pressure, and especially less than 0.5 bar below atmospheric pressure, or even less than 0.3 bar (e.g. 0.05 to 0.3 bar) below atmospheric pressure, and especially about 0.1 bar below atmospheric pressure.
The vacuum is applied through very fine vacuum holes, such as laser bored holes having a diameter of less than 0.5 mm, especially less than 0.3 mm, and particularly about 0.2 mm, through the mold surface of the front mold. The very fine diameter of these vacuum holes, in combination with the very low vacuum being applied, and the buffering effect of the solid foam backing, ensures that the melted skin film material will not be sucked into these vacuum holes, and the finished surface of the skin film on the molded part will not show any nubs or pips of skin material at the locations of the vacuum holes. Instead, the finished textured surface of the molded part will be an exact reproduction of the surface texture provided on the front mold surface, without any distortion and without any vacuum hole marks.
Moreover, as is evident from the above discussion, the objects of the invention have further been achieved using a molding apparatus including a first front mold and a second back mold. The front mold has a mold surface that has been milled to the required contour and then highly polished, and thereafter provided with a surface texture by mechanical engraving, milling, etching, or the like. The front mold includes a vacuum plenum connected to a source of vacuum, and vacuum holes extending from the vacuum plenum through the front mold surface. These vacuum holes particularly include a mechanically bored hole portion, and a laser bored hole portion extending from the mechanically bored hole portion through the front mold surface. The laser bored hole portions have diameters as described above. Both the front mold and the back mold are tempered by flowing cooling water or cooling oil or the like through coolant passages, for example to maintain the molds at a temperature of 50 to 60xc2x0 C.
The back mold substantially mates with the front mold, with a suitable mold gap therebetween, and is movable relative to the front mold. Namely, either one of the molds is movable relative to the other, or both molds are movable relative to each other. The back mold includes pressure medium distribution passages and pressure medium holes, connected to a source of pressure medium, such as pressurized air with a pressure in the range from 1 to 30 bar. The pressure medium holes are provided on all surfaces of the mold surface of the back mold, to ensure a uniform distribution of the pressure medium. A perimeter seal frame provides a seal between the front mold and the back mold around the perimeter thereof. The seal frame is preferably mounted to the back mold with a yielding connection, such as by a spring-loaded slide arrangement. The seal frame can also function as a mechanical stop that limits the closing travel of the molds so as to reliably maintain the intended gap therebetween as mentioned above.
Either the front mold or the back mold may be configured as a female mold, while the respective opposite mold is a mating male mold. Similarly, either the front mold or the back mold may be considered as the negative mold or the positive mold, and either of these molds may be the top mold or the bottom mold. Preferably, the front mold is the top mold, while the back mold is the bottom mold. This mold arrangement corresponds to the cover sheet being oriented with the foam backing on the bottom, carrying the melted skin film material on top of it. This is necessary especially for larger molded parts, because flipping this arrangement xe2x80x9cupside downxe2x80x9d would allow the melted skin film material to flow or drip down from the foam backing that acts as a carrier. The xe2x80x9cupside downxe2x80x9d arrangement can, however, be used successfully for smaller sized parts to be molded.
The inventive apparatus and method further include a heater arrangement, preferably with an infrared heater on top for heating the upper skin film material (e.g. to about 200xc2x0 C.) by infrared radiation heating, and a liquid-cooled metal tempering plate on the bottom to support and keep the foam backing material at a lower temperature (e.g. about 140xc2x0 C.), in a preferred embodiment. The term xe2x80x9caboutxe2x80x9d with reference to temperatures herein means xc2x15xc2x0 C., unless otherwise defined. The cover sheet is carried by a tensioning frame, clamp frame, tentering frame or the like. This frame carries the cover sheet into the heater arrangement, then from there into the forming mold arrangement.